The invention relates to molding machines in general, and more particularly to improvements in apparatus for admitting hardenable plastic or other materials into the dies of injection molding, transfer molding and like machines. Still more particularly, the invention relates to improvements in apparatus which can be used with advantage for admission of heat- and shear-sensitive hardenable materials into the runners of molding machines.
Apparatus of the type to which the present invention pertains are disclosed, for example, in German Pat. No. 32 45 571. They are designed to ensure that the die of a molding machine can be filled with hardenable material without the development of sprue. Such apparatus comprise a housing with an inlet and an outlet for hardenable material and a needle-like valving element which is reciprocable in the housing between an extended position in which the outlet is sealed and a retracted position in which the material which is admitted by way of the inlet can flow into, through and beyond the outlet on its way into the runner of a molding machine. An important requirement which must be satisfied by apparatus of the above outlined character is to ensure that the outlet of the housing is reliably sealed in extended position of the valving element and that the valving element offers little or no resistance to the outflow of hardenable material when it is caused to assume the retracted position.
As a rule, the valving element is reciprocably mounted at the center of the housing and the inlet for admission of hardenable material is located at one side of the valving element. Such inlet admits hardenable material into a chamber which is provided in the housing and completely surrounds the valving element when the latter is caused to assume its extended position. Thus, the valving element at least partially divides the mass of hardenable material in the chamber. This creates problems when the apparatus is to receive certain types of materials which are not likely to ensure satisfactory intermixing of the divided mass in the housing when the valving element is moved to its retracted position. Consequently, different batches of the mass of hardenable material in the housing are maintained under different tension and this results in the development of pronounced boundaries between portions of the molded articles with the likelihood of development of breaks or cracks along such boundaries. Attempts to overcome the just outlined drawbacks of conventional apparatus by intensive heating of the valving element have met with limited success or with no success at all.